Why allocate() and deallocate() has not been called when using self-defined allocator with std::string?

Question

Why allocate() and deallocate() has not been called when using self-defined allocator with std::string?

Here is the code snippet for demo(https://coliru.stacked-crooked.com/a/bcec030e8693f7ae):

#include <cstring>
#include <iostream>
#include <limits>

#define NOMINMAX
#undef max

template <typename T>
class UntrackedAllocator {
public:
    typedef T value_type;
    typedef value_type* pointer;
    typedef const value_type* const_pointer;
    typedef value_type& reference;
    typedef const value_type& const_reference;
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;

public:
    template<typename U>
    struct rebind {
        typedef UntrackedAllocator<U> other;
    };

public:
    inline explicit UntrackedAllocator() {}
    inline ~UntrackedAllocator() {}
    inline UntrackedAllocator(UntrackedAllocator const&) {}
    template<typename U>
    inline explicit UntrackedAllocator(UntrackedAllocator<U> const&) {}

    //    address
    inline pointer address(reference r) {
        return &r;
    }

    inline const_pointer address(const_reference r) {
        return &r;
    }

    //    memory allocation
    inline pointer allocate(size_type cnt,
        typename std::allocator<void>::const_pointer = 0) {
        std::cout << "allocate()" << std::endl;
        T *ptr = (T*)malloc(cnt * sizeof(T));
        return ptr;
    }

    inline void deallocate(pointer p, size_type cnt) {
        std::cout << "deallocate()" << std::endl;
        free(p);
    }

    //   size
    inline size_type max_size() const {
        return std::numeric_limits<size_type>::max() / sizeof(T);
    }

    // construction/destruction
    inline void construct(pointer p, const T& t) {
        new(p) T(t);
    }

    inline void destroy(pointer p) {
        p->~T();
    }

    inline bool operator==(UntrackedAllocator const& a) { return this == &a; }
    inline bool operator!=(UntrackedAllocator const& a) { return !operator==(a); }
};

typedef std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char>> String;

int main() 
{
    String str { "13" };
    String copy = str;
    const char* cstr = str.c_str();
    int out = atoi(cstr);

    std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char> > str1("hello world");

    std::cout << str1 << std::endl;

    std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char> > str2;

    str2 = "hi";

    std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char> > longStr =  str1 + str2;

    std::cout << longStr << std::endl;

}

Here is the outputs(no more indeed):

hello world
hello worldhi